Polymeric hydrogen diffusion barrier, high-pressure storage tank so equipped, method of fabricating a storage tank and method of preventing hydrogen diffusion

ABSTRACT

An electrochemically active hydrogen diffusion barrier which comprises an anode layer, a cathode layer, and an intermediate electrolyte layer, which is conductive to protons and substantially impermeable to hydrogen. A catalytic metal present in or adjacent to the anode layer catalyzes an electrochemical reaction that converts any hydrogen that diffuses through the electrolyte layer to protons and electrons. The protons and electrons are transported to the cathode layer and reacted to form hydrogen. The hydrogen diffusion barrier is applied to a polymeric substrate used in a storage tank to store hydrogen under high pressure. A storage tank equipped with the electrochemically active hydrogen diffusion barrier, a method of fabricating the storage tank, and a method of preventing hydrogen from diffusing out of a storage tank are also disclosed.

GOVERNMENT RIGHTS

[0001] The United States Government has rights in the followinginvention pursuant to Contract No. DE-AC07-991D13727 between the UnitedStates Department of Energy and Bechtel BWXT Idaho, LLC.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] An electrochemically active gas diffusion barrier is disclosed.The electrochemically active barrier may be used in a high-pressurestorage tank to prevent hydrogen from diffusing out of the storage tank.

[0004] 2. State of the Art

[0005] Compressed gases, such as natural gas and hydrogen, are beingdeveloped as alternative fuels to replace gasoline and diesel fuels. Inorder to use compressed gases as fuel sources in vehicles, the vehiclesare modified or redesigned to use these alternative fuel sources. Forinstance, conventional storage tanks for gasoline and diesel fuelscannot withstand high pressures associated with the use of compressedgases. A storage tank of a vehicle that runs on such alternative fuelsis configured with an internal shape to withstand the high pressures.The storage tank is also limited in its size and external shape by thespace, or available storage envelope, under or within the vehicle thatis available for mounting the storage tank. In addition, weight of thestorage tank is desirably kept to a minimum so that it does not increasethe overall weight of the vehicle. Conventional storage tanks forvehicles that run on alternative fuels are bottle-shaped and are mountedto the underside of the vehicle.

[0006] A conformable storage tank for compressed natural gas isdisclosed in U.S. Pat. No. 5,577,630 to Blair et al. (“Blair”), thedisclosure of which is incorporated herein by reference. The conformablestorage tank is formed from a polymeric liner that is purported to beimpermeable to the compressed natural gas stored in the storage tank.The polymeric liner is overwrapped with a composite material to form areinforcement layer over the polymeric liner. The polymeric liner is athin layer of a polyamide, a polyethylene, a polypropylene, apolyurethane, or a blend or copolymer of these materials. The compositematerial is typically a carbon, glass, graphite, aramid, or other fiberbound in a thermoplastic or thermoset epoxy resin. The conformablestorage tank has a normal operating pressure to 3,600 pounds per squareinch (“psi”) and a burst strength of 11,000 psi.

[0007] While the polymeric liner of Blair is used to prevent compressednatural gas from diffusing out of the storage tank, this polymeric lineris not useful to store hydrogen under any significant pressure becausehydrogen has a significantly higher permeability rate through polymersthan natural gas. The permeability rate of hydrogen is up to 80 timesgreater than that of natural gas, depending on the polymeric materialused in the polymeric liner. If hydrogen is stored in a storage tankhaving a polymeric liner similar to that in Blair, hydrogen pressure isgradually lost as the hydrogen diffuses out of the storage tank. Thediffusing hydrogen also affects the bonding between the polymeric linerand the reinforcement layer because the hydrogen weakens the bondbetween the two layers.

[0008] While other materials are less permeable to hydrogen thanpolymers, these materials are not currently feasible for coating apolymeric liner of a storage tank to prevent the diffusion of hydrogenand, in addition, exhibit other undesirable characteristics forvehicular applications. Metals and ceramics are less permeable tohydrogen than polymers, but have a higher stiffness or modulus value. Ifmetal or ceramic coatings are formed on the polymeric liner, thecoatings would crack when the storage tank was pressurized with hydrogendue to the different stiffnesses of the polymeric liner and the coating,causing the more yieldable polymeric liner to flex before the stiffercoating. Amorphous metals or ceramics also have low hydrogenpermeability, but have even higher modulus values. Furthermore, theseamorphous metals and ceramics are not easily formed in thin coatings.Coating techniques, such as sputtering or chemical vapor deposition, areproblematic because sputtering equipment is not available for coatingthe inside of a storage tank and chemical vapor deposition requires hightemperatures that would damage or destroy the polymeric liner. Usingmetals and ceramics to coat the polymeric liner is further problematicbecause such materials are heavier than polymers and drasticallyincrease the overall weight of a storage tank.

[0009] It would be desirable to store high-pressure hydrogen in astorage tank that is effectively impermeable to hydrogen diffusion. Itwould be further desirable to reduce the permeability of hydrogenthrough polymeric liners so that storage tanks currently used to storecompressed natural gas may be modified to store hydrogen.

BRIEF SUMMARY OF THE INVENTION

[0010] The present invention comprises a hydrogen diffusion barrier toprevent hydrogen from diffusing through a polymer substrate. Thehydrogen diffusion barrier comprises three polymer layers and an energysource. The three polymer layers comprise an anode layer, a cathodelayer, and an interposed electrolyte layer, which is conductive toprotons and impermeable to hydrogen. The hydrogen diffusion barrier usesan electrochemically active structure to prevent the hydrogen fromdiffusing through the polymer substrate. If hydrogen passes through theelectrolyte layer, a catalytic material present adjacent the interfaceof the electrolyte layer and the anode layer catalyzes anelectrochemical reaction to convert the hydrogen to protons andelectrons. The protons and electrons are transported to the cathodelayer and reacted to form hydrogen. A catalytic material presentadjacent the interface of the cathode layer and the electrolyte layermay be used to enhance the formation of hydrogen.

[0011] The present invention also comprises a storage tank for storinghigh-pressure hydrogen. The storage tank comprises a polymer substratebonded to a reinforcement layer and a hydrogen diffusion barrier bondedto the polymer substrate. The hydrogen diffusion barrier comprises ananode layer and a cathode layer, each of which comprises a polymermaterial permeable to hydrogen. The-hydrogen-diffusion barrier alsocomprises an energy source and an electrolyte layer comprising a polymermaterial conductive to protons and impermeable to hydrogen. Theelectrolyte layer is disposed between the anode layer and the cathodelayer and a catalytic material is present adjacent the interface betweenthe electrolyte layer and at least one of the anode layer and thecathode layer, and preferably adjacent both interfaces. The hydrogendiffusion barrier utilizes this electrochemically active structure toprevent hydrogen from diffusing through the polymer substrate and out ofthe storage tank.

[0012] The present invention further comprises a method of preventinghydrogen from diffusing out of a storage tank. The method comprisesproviding a storage tank having a polymer substrate bonded to asurrounding reinforcement layer and a hydrogen diffusion barrier bondedto and within the polymer substrate. The hydrogen diffusion barriercomprises an anode layer, a cathode layer, an interposed electrolytelayer, and an energy source. The storage tank is pressurized withhydrogen and any hydrogen that diffuses through the electrolyte layer istransported back to the storage tank using an electrochemically activestructure including a catalytic material to catalyze a reaction fromhydrogen to protons and electrons. The protons and electrons aretransported to the cathode layer where they react in the presence of acatalytic material to form hydrogen, which diffuses back into thestorage tank. The energy source provides a sufficient amount of energyto transport the electrons to the cathode layer while the protonsdiffuse through the electrolyte layer back to the cathode layer.

[0013] The present invention still further includes a method offabricating a storage tank to store high-pressure hydrogen. The methodcomprises providing a polymer substrate and bonding a reinforcementlayer to a first surface of the polymer substrate. A hydrogen diffusionbarrier susceptible to electrical stimulation to becomeelectrochemically active is formed on a second surface of the polymersubstrate. The hydrogen diffusion barrier comprises a polymeric anodelayer permeable to hydrogen, a polymeric electrolyte layer conductive toprotons and substantially impermeable to hydrogen, and a polymericcathode layer permeable to hydrogen. The three polymer layers of thehydrogen diffusion barrier may be formed by dip-coating orspray-coating. A catalytic material may be disposed adjacent theinterfaces between the polymeric layers to facilitate dissociation ofhydrogen into protons and electrons and to facilitate formation ofhydrogen therefrom. An energy source is operably coupled to the anodelayer and the cathode layer to render the hydrogen diffusion barrierelectrochemically active.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0014] In the drawings, which illustrate what is currently considered tobe the best mode for carrying out the invention:

[0015]FIG. 1 is a cross-sectional view of a conformable storage tankhaving a hydrogen diffusion barrier of the present invention;

[0016]FIG. 1A is an enlarged view of the circled region in FIG. 1;

[0017]FIGS. 2 and 3 illustrate alternative embodiments of the hydrogendiffusion barrier; and

[0018] FIGS. 4A-4C illustrates the movement of hydrogen, protons, andelectrons through the hydrogen diffusion barrier of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0019] An electrochemically active barrier to prevent diffusion ofhydrogen through a polymer substrate is disclosed. The electrochemicallyactive barrier is bonded to the polymer substrate and uses anelectrically stimulated structure to electrochemically preventhigh-pressure hydrogen from reaching the polymer substrate and diffusingtherethrough. The electrochemically active barrier or hydrogen diffusionbarrier is used in a storage tank lined with the polymer substrate tostore high-pressure hydrogen. The hydrogen diffusion barrier issubstantially impermeable to hydrogen and is also able to return anyhydrogen that diffuses out of the storage tank back to the storage tankbefore reaching the polymer substrate using an electrochemical reaction.

[0020] The hydrogen diffusion barrier may be bonded to the polymersubstrate. The hydrogen diffusion barrier may have a low permeability tohydrogen so that hydrogen does not diffuse through the hydrogendiffusion barrier and the polymer substrate. The hydrogen diffusionbarrier may also have a stiffness or modulus value similar to that ofthe polymer substrate so that excessive stress resulting from thepressurized hydrogen does not cause dissimilar strain tendencies betweenthe hydrogen diffusion barrier and the polymer substrate, which mayinitiate cracking when the storage tank is pressurized. In addition, thehydrogen diffusion barrier may be formed using coating techniques thatproduce a hermetic coating that is free of pin holes. Furthermore, thehydrogen diffusion barrier may be formed from low cost, low weightpolymeric materials so that the added cost and weight is minimal.

[0021] The hydrogen diffusion barrier 2 may include three polymer layersand an energy source 4, as shown in FIG. 1A, comprising an enlargementof the circled region of FIG. 1. Two of the polymer layers may beelectrically conductive while the third polymer layer may beelectrically insulative, comprising a dielectric material. Theelectrically conductive polymer layers may form electrode layers 6, suchas an anode layer 6′ and a cathode layer 6″. The anode layer 6′ andcathode layer 6″ may be formed from polymeric materials that are eitherporous or dense. As used herein, the term “porous” refers to a polymericmaterial that is permeable to very small diameter atoms or molecules(i.e. hydrogen or helium). The term “dense” refers to a polymericmaterial that is substantially impermeable (or has a low level ofpermeability) to large diameter gaseous atoms or molecules (i.e. argonor nitrogen) but has a higher level of permeability to very smalldiameter atoms or molecules (i.e. hydrogen or helium). It is currentlypreferred that the anode layer 6′ is a dense polymeric material and thecathode layer 6″ is a porous polymeric material. However, it is alsocontemplated that the anode layer 6′ may be a porous polymeric materialwhile the cathode layer 6″ may be a dense polymeric material. In thissituation, the porous anode layer 6′ may facilitate the removal ofhydrogen from an interface between the electrolyte layer 8 and the anodelayer 6′ when the protons are recombined with electrons to formhydrogen. Additionally, both of the electrode layers 6 may be densepolymeric materials.

[0022] The electrode layers 6 used in the hydrogen diffusion barrier 2may be sufficiently permeable to hydrogen so that a small portion of thetotal hydrogen passes through the layers. The electrode layers 6 mayhave similar hydrogen permeabilities to those of polyethylene orpolyvinyl chloride, both of which are polymers and have hydrogenpermeabilities of 15.7×10⁻³ CF mil/ft²-day-atm and 13.7×10⁻³ CFmil/ft²-day-atm, respectively. While the electrode layers 6 arepermeable to hydrogen, the overall hydrogen diffusion barrier 2 may, inoperation, be substantially impermeable to hydrogen.

[0023] Each of the two electrode layers 6 may be formed from the samepolymeric material or from a different polymeric material. The polymericmaterial used in the electrode layers 6 may be similar to the polymericmaterial used in an electrode of a conventional proton exchange membrane(“PEM”), which is also known as a hydrogen pump. PEMs are known in theart to separate, transport, or supply hydrogen. Electrically conductivepolymers that may be used in the electrode layers 6 include, but are notlimited to, polyaniline (“PANI” or “PA”), polypyrrole (“PPy”), blends ofPA/PPy, and poly(3,4-ethylenedioxythiophene)polystyrenesulfonate(“PEDOT-PSS”).

[0024] The electrically insulative layer may be an electrolyte layer 8that is formed from a dense, proton-conducting electrolyte that isdesirably substantially impermeable to hydrogen. Such aproton-conducting, substantially hydrogen impermeable electrolyte may bea “dry” electrolyte material. As used herein, a dry electrolyte materialis an electrolyte material that is not hydrated or associated withwater.

[0025] The proton-conducting, substantially hydrogen-impermeableelectrolyte used in the electrolyte layer 8 may be formed of aconventional electrolytic material. For example, polybenzimidazole maybe used as the proton-conducting, substantially hydrogen-impermeableelectrolyte. The electrolytic material may also include a microporouspolymer matrix filled with an inorganic proton conductor, such astungstophosphoric acid. Electrolytic materials commonly used as amembrane in a PEM may also be used. For example, the proton-conductingelectrolyte may include, but is not limited to, a perfluorinatedsulfonic acid derivative, a polytetrafluoroethylene, atetrafluoroethylene/(perfluoroalkyl) vinyl ether copolymer, atetrafluoroethylene/hexafluoropropylene copolymer, apoly(trifluorostyrene) copolymer, a sulfonated poly(aryl ether ketone),a sulfonated polyaromatic based system, or a sulfonatedpoly(2,6-dimethyl-1,4-phenylene oxide). These electrolytes arecommercially available as NAFION® membranes from E. I. du Pont deNemours and Company (Wilmington, Del.), GORE-SELECT® membranes from W.L. Gore (Newark, Del.), ACIPLEX® from Asahi Kasei Corp. (Tokyo, Japan),and FLEMION from Asahi Glass Co. (Tokyo, Japan).

[0026] As shown in FIG. 1, the electrolyte layer 8 may be sandwichedbetween the anode layer 6′ and the cathode layer 6″. As notedpreviously, the electrolyte layer 8 may be substantially impermeable tohydrogen (“H₂”) but may conduct protons (“H⁺”). In other words, theelectrolyte layer 8 may have a very low permeability to hydrogen buthave a high proton conductivity. However, the proton-conductingelectrolyte used in the electrolyte layer 8 may not require the samedegree of proton conductivity that is necessary in conventional PEMsthat are used in fuel cells.

[0027] Since the three layers of the hydrogen diffusion barrier 2 areformed from polymers, which weigh less than metal or ceramic materialsthat are impermeable to hydrogen, the weight of the hydrogen diffusionbarrier 2 may be minimal. To keep the weight of the hydrogen diffusionbarrier 2 minimal, the thickness of the hydrogen diffusion barrier 2 mayalso be minimized. The polymer layers may each be approximately 2-25 μmthick, with the hydrogen diffusion barrier 2 having a total thickness ofapproximately 6-75 μm. However, the thickness of each polymer layerdepends on the viscosity of a monomeric precursor to the polymer layerand the temperature at which the monomeric precursor is applied. Byusing a minimal amount or thickness of polymeric material, the cost ofthe hydrogen diffusion barrier 2 may also be minimized.

[0028] The hydrogen diffusion barrier 2 may include a catalyst in theform of a catalytic metal 20 located between either or both of theelectrode layers 6 and the electrolyte layer 8 as an element of theelectrochemical structure for preventing the hydrogen from diffusing.For instance, a catalyst layer formed from a catalytic metal 20 may bepresent proximate the interface between the anode 6′ and the electrolytelayer 8 and/or between the cathode 6″ and the electrolyte layer 8, asshown in FIG. 2. It is currently preferred that a catalytic metal 20 belocated proximate both interfaces. An electrochemical reaction may occurat each of two junctions, or phase boundaries, where hydrogen, one ofthe electrode layers 6, and the electrolyte layer 8 interface. Thecatalyst layer may be formed from a catalytic metal 20 such as a metal,metal oxide, metal alloy, or other metal compound, including mixtures ofmetals in groups IV-B, V-B, VI-B, VII-B, VIII, I-B, II-B, III-B, IV-A,and V-A of the Periodic Chart of the Elements. The catalytic metal 20may include the metals, metal oxides, metal alloys, or other metalcompounds of titanium, zirconium, vanadium, niobium, chromium,molybdenum, tungsten, manganese, rhenium, ruthenium, osmium, cobalt,rhodium, iridium, nickel, palladium, platinum, copper, silver, gold,zinc, cadmium, yttrium, tin, and lead. It is currently preferred thatthe catalytic metal 20 be nanometer-sized particles of platinum(platinum black) or ruthenium.

[0029] The catalytic metal 20 nay also be dispersed in the polymericmaterial of the anode layer 6′ or cathode layer 6″ rather than beingpresent as an identifiable, discrete layer. The catalyst layer ordispersion of catalytic metal 20 of the hydrogen diffusion barrier 2 maybe formed using known techniques of forming catalyst layers inconventional PEMs. However, the amount of catalytic metal 20 present inthe hydrogen diffusion barrier 2 may be less than the amount used in thecatalyst layer of a conventional PEM because the hydrogen diffusionbarrier 2 is not used to generate energy. While the catalytic metal 20is described herein as an element of the electrochemically activestructure, it is also contemplated that other electrochemically activeelements for catalyzing the reaction between hydrogen, protons, andelectrons may be used.

[0030] While it is currently preferred that the catalytic metal 20 bepresent in the hydrogen diffusion barrier 2, the electrically conductivepolymers used in the electrode layers 6 may also provide the necessarycatalytic activity. If these polymers exhibit catalytic activity, thecatalytic metal 20 may not be necessary in the hydrogen diffusionbarrier 2.

[0031] The electrochemically active barrier 2 may include an energysource 4 that is operably coupled with the electrode layers 6sandwiching the electrolyte layer 8. The energy source 4 may provide alow D.C. voltage, and very little current flow, to prevent hydrogen fromdiffusing out of the storage tank. The amount of energy that isnecessary to prevent the diffusion of hydrogen is extremely low and,therefore, the energy source 4 may be a lithium battery, such as aconventional watch or calculator battery.

[0032] The hydrogen diffusion barrier 2 is an active barrier to hydrogenin that it uses electrochemical reactions (proximate the interfaces 16,18 of the electrode layers 6 and the electrolyte layer 8) and theapplied voltage to prevent the hydrogen from diffusing. If the appliedvoltage is not present to stimulate the hydrogen diffusion barrier, thehydrogen diffusion barrier 2 may be inoperative. In other words, thevoltage produced by the energy source 4 may be present at all times. If,for example, the energy source 4 is not working, such as if the batteryor batteries comprising the energy source 4 are depleted, hydrogen inthe storage tank may diffuse through the hydrogen diffusion barrier 2and out of the storage tank.

[0033] The hydrogen diffusion barrier 2 may be used in a conformablestorage tank 10 used in a vehicle, such as an automobile, truck, or bus.The hydrogen diffusion barrier 2 prevents high-pressure hydrogen 22(FIG. 1) from diffusing out of the conformable storage tank 10. Whilethe embodiments described and illustrated herein show that the hydrogendiffusion barrier 2 is used in conformable storage tank 10, the hydrogendiffusion barrier 2 may also be used in any type of storage tanks usedto store high-pressure hydrogen. In addition, the hydrogen diffusionbarrier 2 may be used in storage tanks for storing hydrogen forapplications other than as an alternative fuel for vehicles. Thehydrogen diffusion barrier 2 may also be adapted to prevent compressedgases other than hydrogen from diffusing out of a storage tank. Forexample, the polymeric materials of the hydrogen diffusion barrier 2 maybe selected to be substantially impermeable to another compressed gasbut yet be able to dissociate the gas into an ionized form and transportan ionized form of that compressed gas for reformation. In addition, thecatalytic metal 20 may be selected to catalyze an electrochemicalreaction that uses the compressed gas or the ionized form of thecompressed gas as a product or reactant.

[0034] The storage tank 10 includes the polymer substrate 12, areinforcement layer 14, and the hydrogen diffusion barrier 2. In oneembodiment, the polymer substrate 12 is located between thereinforcement layer 14 and the hydrogen diffusion barrier 2, as depictedin FIG. 1. In another embodiment, the hydrogen diffusion barrier 2 islocated between the polymer substrate 12 and the reinforcement layer 14,as illustrated in FIG. 3. The polymer substrate 12 may be athermoplastic or thermosetting polymer, such as a polyamide, apolyethylene, a polypropylene, a polyurethane, and a blend or copolymerthereof. The reinforcement layer 14 may be a composite material such asa carbon, glass, graphite, aramid, or other fiber bound in athermoplastic or thermoset epoxy resin. For instance, a two-part epoxyresin may be mixed and carbon fibers impregnated with the resin. Thereinforcement layer 14 may be wound in the form of resin-coatedfilaments or bundles or tapes of onto the polymer substrate 12 or thehydrogen diffusion barrier 2, as known in the art. In anotherembodiment, the polymer substrate 12 is a cross-linked polyethylene or anylon, such as Nylon 6, Nylon 11, Nylon 12, Nylon 66, or Nylon 610 andthe reinforcement layer 14 is a graphite fiber bound in a two-part epoxyresin.

[0035] In the embodiment illustrated in FIG. 1, a first surface of thehydrogen diffusion barrier 2 may be bonded to the polymer substrate 12by bonding one of the polymer layers to a surface of the polymersubstrate 2. The first surface may be a surface of one of the electrodelayers 6. A second surface of the hydrogen diffusion barrier 2 may be incontact with the hydrogen 22 stored in the storage tank 10. The secondsurface may be a surface of the other electrode layer 6. In theembodiment shown in FIG. 3, the polymer substrate 12 may contact thehydrogen 22 stored in the storage tank 10.

[0036] The hydrogen diffusion barrier 2 and the polymer substrate 12 mayhave similar modulus or stiffness values so that they do not crack whenthe storage tank 10 is pressurized. Rather, the hydrogen diffusionbarrier 2 may pass the load or stress to the polymer substrate 12, whichultimately passes the load to the stiffer reinforcement layer 14. Sincethe polymer substrate 12 may be formed of plastics, such as polyethyleneor nylon, which have similar modulus values to the polymeric materialsused in the hydrogen diffusion barrier 2, neither the hydrogen diffusionbarrier 2 nor the polymer substrate 12 will crack when the storage tankis pressurized as they will flex at substantially the same rate untilsupported by the stiffer reinforcement layer 14.

[0037] The three polymer layers of the hydrogen diffusion barrier 2 maybe formed by techniques that produce a thin, hermetic, pin-hole freecoating of the polymeric material. Each of the polymer layers may beformed individually, with a first polymer layer formed on the polymersubstrate 12. The first polymer layer may be bonded to the polymersubstrate and a second polymer layer formed and bonded on the underlyingfirst polymer layer. A third polymer layer may be formed on theunderlying second polymer layer and bonded thereto. The first polymerlayer may be one of the electrode layers 6, the second polymer layer maybe the electrolyte layer 8, and the third polymer layer may be the otherelectrode layer 6. Each of the polymer layers may be formed respectivelyin situ on the polymer substrate 12 or the underlying polymer layers bypolymerizing or curing a monomeric precursor of each of the polymerlayers. The polymer layers may be formed by conventional techniquesincluding, but not limited to, dip-coating or spray-coating.

[0038] To form the first polymer layer by dip-coating, the storage tank10 may be dipped in a solution of the monomeric precursor of the firstpolymer layer so that the monomeric precursor contacts the innersurface, or polymer substrate 12, of the storage tank 10. The storagetank 10 is spun so that the monomeric precursor coats the polymersubstrate 12. Excess monomeric precursor is removed. The monomericprecursor coating the polymer substrate 12 is polymerized to form thefirst polymer layer. The monomeric precursor may be polymerized by heat,an infrared source, an ultraviolet source, or a combination thereof, tobond the first polymer layer to the polymer substrate 12, depending onthe type of monomeric precursor that is used. Furthermore, anappropriate catalyst for polymerizing and thus bonding the first polymerlayer to the substrate can be used, either separately, or in combinationwith a heat source, an infrared source, an ultraviolet source or acombination thereof. Once the first polymer layer is formed, the secondand third polymer layers may be formed in substantially the same way.

[0039] If the catalytic metal 20 is present as a layer, the catalystlayer may be formed by adding a small amount of nanometer-sizedparticles of the catalytic metal 20 into the electrode layers 6. Aspreviously mentioned, the amount of catalytic metal 20 present in theelectrode layers 6 may be less than the amount used in a conventionalPEM. The catalytic metal 20 may be mixed into the monomeric precursorsof the electrode layers 6 before the electrode layers 6 are formed.Alternatively, a thin layer of the polymeric material containing thecatalytic metal 20 may be applied on either side of the electrolytelayer 8. For example, the thin layer may be formed between the firstpolymer layer and the second polymer layer and between the secondpolymer layer and the third polymer layer. Since these thin layers areformed in direct contact with the electrolyte layer 8, the catalyticmetal 20 may be present at the interfaces 16 and 18, where the catalyticmetal 20 is most effective.

[0040] The polymer layers may also be formed by spray-coating. The firstpolymer layer may be formed by spraying its monomeric precursor on thepolymer substrate 12. The monomeric precursor may be sprayed byinserting a nozzle in the storage tank 10, which may conventionally havea narrow opening (approximately 1-2 inches in diameter) at one end. Thenozzle may be of a sufficiently small size to fit through the narrowopening. The monomeric precursor may be polymerized by heat, an infraredsource, or an ultraviolet source to bond the first polymer layer to thepolymer substrate 12, again depending upon the polymer density employed.The second and third polymer layers may be formed over the underlyingfirst polymer layer in substantially the same way.

[0041] In addition to dip-coating and spray-coating, it is alsocontemplated that each of the polymer layers may be formed by extrudingor molding each layer of the polymeric material of the combined layersinto a desired shape conformable to the interior of the storage tank andpressing the polymeric material to the polymer substrate 12 using abladder inflated within the storage tank. Extrusion and moldingtechniques are known in the art and, therefore, are not described indetail herein.

[0042] The storage tank 10 having the hydrogen diffusion barrier 2 maybe pressurized with hydrogen to between approximately 5,000 and 10,000psi. The storage tank 10 may have a burst strength of approximately10,000 psi and a normal operating pressure of approximately 5,000-8,000psi. When the storage tank 10 is pressurized, the hydrogen may diffuseinto the cathode layer 6″ as illustrated in FIG. 4A. Upon contacting theinterface 18 between the cathode layer 6″ and the electrolyte layer 8, amajority of the hydrogen 22 remains in the cathode layer 6″ because theelectrolyte layer 8 is substantially impermeable to hydrogen. Since thecathode layer 6″ is porous, the hydrogen 22 may also diffuse out of thecathode layer 6″ and back into the storage tank 10. However, a verysmall portion of the hydrogen 22 may pass into the electrolyte layer 8and diffuse through the electrolyte layer 8 to reach the anode layer 6′.The hydrogen 22 may be converted (disassociated) into protons 24 andelectrons 26 proximate interface 16 by the catalytic metal 20 in thereaction H₂→2H⁺+2e⁻. The electrons 26 are conducted through anelectrically conductive material of conductive traces 28 through anexternal circuit of the energy source 4 to the cathode layer 6″, whilethe protons 24 are transported back through the proton-permeableelectrolyte layer 8 to the cathode layer 6″, as shown in FIG. 4B. Thehydrogen permeability and proton conductivity of the electrolyte layer 8may be related such that one-half of the proton flux through theelectrolyte layer 8 is approximately equal to the hydrogen flux acrossthe electrolyte layer 8, thereby providing a net zero flux of hydrogenand protons across the electrolyte layer 8. The energy source 4 producesa voltage sufficient to transport the electrons 26 to the cathode layer6″. The protons 24 and transported electrons 26 react proximate theinterface 18 in the reaction 2H⁺+2e⁻→H₂ to form hydrogen 22, suchreaction being facilitated by the presence of catalytic metal 20proximate interface 18, the hydrogen then diffusing back into thestorage tank 10, as shown in FIG. 4C.

[0043] Since only a small portion of the hydrogen 22 passes through theelectrolyte layer 8, a partial pressure of hydrogen at the interface 16of the anode layer 6′ and the electrolyte layer 8, referred to herein asPH₂ ^(II), may be low. PH₂ ^(II) may be between approximately 1×10⁻¹⁰and 1×10⁻² atmospheres (“atm”). Since only a low partial pressure ofhydrogen is present at interface 16, the voltage necessary to transportthe protons 24 and electrons 26 to the cathode layer 6″ may be quitelow. The required applied voltage may be derived from the Nernstequation:

E=ΔG/nF=RT/nF×ln(PH₂ ^(I)/PH₂ ^(II))

[0044] where E is the applied voltage, ΔG is the free energy of thereaction, n is the number of electrons involved in the reaction, F isFaraday's constant (96,520 C/mole), R is the universal gas constant(8.314 J/g mole K), T is temperature, and PH₂ ^(I) is the partialpressure of hydrogen in the storage tank.

[0045] Thus, the energy required to transport the electrons 26 to thecathode layer 6″ may be calculated using the Nernst equation. For sakeof example only, the following assumptions may be made: the reaction maybe run at 300K, the hydrogen in the storage tank (PH₂ ^(I)) may bepressurized to 200 atm (2939.4 psi), and the desired partial pressure ofhydrogen (PH₂ ^(II)) at the interface 16 may be 1×10⁻¹⁰ atm. For theelectrochemical reaction converting hydrogen to protons and electrons,the number of electrons involved in the reaction, or n, is 2. Usingthese assumptions, the applied energy is 0.366 volts, which is less thanthe voltage which, may be derived from a conventional lithium watch orcalculator battery.

[0046] The present invention encompasses a hydrogen diffusion barrierthat prevents hydrogen from diffusing out of a storage tank, and astorage tank incorporating the hydrogen diffusion barrier. The hydrogendiffusion barrier includes three polymer layers and an energy source anduses an electrochemically active structure to prevent the hydrogen fromdiffusing. A method of preventing hydrogen from diffusing out of thestorage tank is also encompassed by the present invention, as is amethod of fabricating the storage tank.

[0047] While the invention may be susceptible to various modificationsand alternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

What is claimed is:
 1. An electrochemically active hydrogen diffusionbarrier comprising: an anode layer and a cathode layer, each layercomprising a polymer material that is permeable to hydrogen; anelectrolyte layer disposed between the anode layer and the cathode layerand comprising a polymer material that is conductive to protons andsubstantially impermeable to hydrogen; a voltage source operably coupledto the anode layer and the cathode layer; and a catalytic materialproximate an interface between at least one of the anode layer and thecathode layer and the electrolyte layer.
 2. The electrochemically activehydrogen diffusion barrier of claim 1, wherein the catalytic materialcomprises a catalytic metal proximate the interface of the electrolytelayer and the anode layer.
 3. The electrochemically active hydrogendiffusion barrier of claim 2, wherein the catalytic metal is selectedfrom the group consisting of platinum, ruthenium, tungsten, molybdenum,and tin.
 4. The electrochemically active hydrogen diffusion barrier ofclaim 2, wherein the catalytic metal proximate the interface of theelectrolyte layer and the anode layer is active to produce protons andelectrons from hydrogen.
 5. The electrochemically active hydrogendiffusion barrier of claim 4, wherein the voltage source operablycoupled to the anode layer and to the cathode layer comprises anelectron transport path for produced electrons.
 6. The electrochemicallyactive hydrogen diffusion barrier of claim 1, wherein the catalyticmaterial comprises a catalytic metal proximate the interface of theelectrolyte layer and the cathode layer.
 7. The electrochemically activehydrogen diffusion barrier of claim 6, wherein the catalytic metal isselected from the group consisting of platinum, ruthenium, tungsten,molybdenum, and tin.
 8. The electrochemically active hydrogen diffusionbarrier of claim 6, wherein the catalytic metal proximate the interfaceof the electrolyte layer and the cathode layer is active to producehydrogen from protons and electrons.
 9. The electrochemically activehydrogen diffusion barrier of claim 1, wherein the polymer material ofthe anode layer and the cathode layer is either a dense polymer or aporous polymer.
 10. The electrochemically active hydrogen diffusionbarrier of claim 1, wherein the polymer material of the electrolytelayer comprises a dense, proton-conducting electrolyte.
 11. Theelectrochemically active hydrogen diffusion barrier of claim 1, whereinthe polymer material of the electrolyte layer comprises a dryelectrolyte material.
 12. The electrochemically active hydrogendiffusion barrier of claim 1, wherein the polymer material of theelectrolyte layer is selected from the group consisting ofpolybenzimidazole, a microporous polymer matrix filled with an inorganicproton conductor, a perfluorinated sulfonic acid derivative, apolytetrafluoroethylene, a tetrafluoroethylene/(perfluoroalkyl) vinylether copolymer, a tetrafluoroethylene/hexafluoropropylene copolymer, apoly(trifluorostyrene) copolymer, a sulfonated poly(aryl ether ketone),a sulfonated polyaromatic based system, and a sulfonatedpoly(2,6-dimethyl-1,4-phenylene oxide).
 13. The electrochemically activehydrogen diffusion barrier of claim 1, wherein the catalytic materialcomprises a catalytic metal proximate the respective interfaces betweenthe anode layer and the electrolyte layer and the cathode layer and theelectrolyte layer.
 14. The electrochemically active hydrogen diffusionbarrier of claim 13, wherein the catalytic metal is selected from thegroup consisting of platinum, ruthenium, tungsten, molybdenum, and tin.15. The electrochemically active hydrogen diffusion barrier of claim 13,wherein the catalytic metal proximate the interface of the electrolytelayer and the anode layer is active to produce protons and electronsfrom hydrogen.
 16. The electrochemically active hydrogen diffusionbarrier of claim 13, wherein the voltage source operably coupled to theanode layer and to the cathode layer comprises an electron transportpath for produced electrons.
 17. A storage tank for storinghigh-pressure hydrogen, comprising: a polymer substrate bonded to areinforcement layer; and an electrochemically active hydrogen diffusionbarrier, comprising: an anode layer and a cathode layer, each layercomprising a polymer material that is permeable to hydrogen; anelectrolyte layer disposed between the anode layer and the cathode layerand comprising a polymer material that is conductive to protons andsubstantially impermeable to hydrogen; wherein the anode layer, thecathode layer and the electrolyte layer are conformably bonded to thepolymer substrate; a voltage source operably coupled to the anode layerand the cathode layer; and a catalytic material proximate an interfacebetween at least one of the anode layer and the cathode layer and theelectrolyte layer.
 18. The storage tank of claim 17, wherein thecatalytic material comprises a catalytic metal at the interface of theelectrolyte layer and the anode layer.
 19. The storage tank of claim 18,wherein the catalytic metal is selected from the group consisting ofplatinum, ruthenium, tungsten, molybdenum, and tin.
 20. The storage tankof claim 18, wherein the catalytic metal proximate the interface of theelectrolyte layer and the anode layer is active to produce protons andelectrons from hydrogen.
 21. The storage tank of claim 20, wherein thevoltage source operably coupled to the anode layer and to the cathodelayer comprises an electron transport path for produced electrons. 22.The storage tank of claim 17, wherein the catalytic material comprises acatalytic metal proximate the interface of the electrolyte layer and thecathode layer.
 23. The storage tank of claim 22, wherein the catalyticmetal is selected from the group consisting of platinum, ruthenium,tungsten, molybdenum, and tin.
 24. The storage tank of claim 22, whereinthe catalytic metal proximate the interface of the electrolyte layer andthe cathode layer is active to produce hydrogen from protons andelectrons.
 25. The storage tank of claim 17, wherein the polymermaterial of the anode layer and the cathode layer comprises either adense polymer or a porous polymer.
 26. The storage tank of claim 17,wherein the polymer material of the electrolyte layer comprises a dense,proton-conducting electrolyte.
 27. The storage tank of claim 17, whereinthe polymer material of the electrolyte layer comprises a dryelectrolyte material.
 28. The storage tank barrier of claim 17, whereinthe polymer material of the electrolyte layer is selected from the groupconsisting of polybenzimidazole, a microporous polymer matrix filledwith an inorganic proton conductor, a perfluorinated sulfonic acidderivative, a polytetrafluoroethylene, atetrafluoro-ethylene/(perfluoroalkyl) vinyl ether copolymer, atetrafluoroethylene/hexafluoropropylene copolymer, apoly(trifluorostyrene) copolymer, a sulfonated poly(aryl ether ketone),a sulfonated polyaromatic based system, and a sulfonatedpoly(2,6-dimethyl-1,4-phenylene oxide).
 29. The storage tank of claim17, wherein the catalytic material comprises a catalytic metal at theinterfaces between the anode layer and the electrolyte layer and thecathode layer and the electrolyte layer.
 30. The storage tank of claim29, wherein the catalytic metal is selected from the group consisting ofplatinum, ruthenium, tungsten, molybdenum, and tin.
 31. The storage tankof claim 29, wherein the catalytic metal proximate the interface of theelectrolyte layer and the anode layer is active to produce protons andelectrons from hydrogen.
 32. The storage tank of claim 29, wherein thecatalytic metal proximate the interface of the electrolyte layer and thecathode layer is active to produce hydrogen from protons and electrons.33. The storage tank of claim 29, wherein the voltage source operablycoupled to the anode layer and to the cathode layer comprises anelectron transport path for produced electrons.
 34. The storage tank ofclaim 17, wherein the storage tank contains hydrogen at a pressure ofbetween approximately 5,000 and 10,000 psi.
 35. The storage tank ofclaim 17, wherein the polymer substrate and the anode layer, the cathodelayer and the electrolyte layer of the hydrogen diffusion barrier havesimilar modulus values.
 36. The storage tank of claim 17, wherein eachof the anode layer, the cathode layer, and the electrolyte layer arehermetic, pin-hole free layers.
 37. A method of preventing hydrogen fromdiffusing out of a storage tank, comprising: providing a storage tankcomprising: a polymer substrate bonded to a reinforcement structure; andan electrochemically active hydrogen diffusion barrier, comprising: ananode layer and a cathode layer each comprising a polymer material thatis permeable to hydrogen; an electrolyte layer disposed between theanode layer and the cathode layer and comprising a polymer material thatis conductive to protons and substantially impermeable to hydrogen;wherein the anode layer, the cathode layer and the electrolyte layer areconformably bonded to the polymer substrate; a voltage source operablycoupled to the anode layer and the cathode layer; and a catalyticmaterial proximate an interface of at least one of the anode layer andthe cathode layer and the electrolyte layer; filling the storage tankwith hydrogen at a pressure in excess of one atmosphere; andtransporting any hydrogen that diffuses outwardly through theelectrolyte layer back to the storage tank using the electrochemicallyactive hydrogen diffusion barrier.
 38. The method of claim 37, whereintransporting hydrogen that diffuses outwardly through the electrolytelayer back to the storage tank using the electrochemically activehydrogen diffusion barrier comprises using the catalytic material toconvert hydrogen to protons and electrons at the passage through theelectrolyte layer.
 39. The method of claim 38, wherein transportinghydrogen that diffuses outwardly through the electrolyte layer back tothe storage tank using the electrochemically active hydrogen diffusionbarrier comprises transporting protons through the electrolyte layer tothe cathode layer and conducting the electrons through the voltagesource to the cathode layer.
 40. The method of claim 39, whereintransporting hydrogen that diffuses outwardly through the electrolytelayer back to the storage tank using the electrochemically activehydrogen diffusion barrier comprises reacting protons and electrons inthe cathode layer to form hydrogen.
 41. The method of claim 37, whereinfilling the storage tank with hydrogen at a pressure in excess of oneatmosphere comprises filling the storage tank with hydrogen to betweenapproximately 5,000 and 10,000 psi.
 42. A method of fabricating astorage tank to store high-pressure hydrogen, comprising: providing apolymer substrate; bonding a reinforcement structure to a first surfaceof the polymer substrate; and forming a hydrogen diffusion barrier on asecond surface of the polymer substrate, the hydrogen diffusion barriercomprising a polymeric anode layer permeable to hydrogen, a polymericelectrolyte layer conductive to protons and substantially impermeable tohydrogen, and a polymeric cathode layer permeable to hydrogen.
 43. Themethod of claim 42, wherein forming a hydrogen diffusion barrier on asecond surface of the polymer substrate comprises bonding the polymericanode layer to the second surface of the polymer substrate.
 44. Themethod of claim 43, wherein forming a hydrogen diffusion barrier on asecond surface of the polymer substrate comprises dipping the storagetank into a solution of a monomeric precursor to the polymeric anodelayer and polymerizing the monomeric precursor to form the polymericanode layer.
 45. The method of claim 42, wherein forming a hydrogendiffusion barrier on a second surface of the polymer substrate comprisesbonding the polymeric electrolyte layer to the polymeric anode layer.46. The method of claim 45, wherein forming a hydrogen diffusion barrieron a second surface of the polymer substrate comprises dipping thestorage tank into a solution of a monomeric precursor to the polymericelectrolyte layer and polymerizing the monomeric precursor to form thepolymeric electrolyte layer.
 47. The method of claim 42, wherein forminga hydrogen diffusion barrier on a second surface of the polymersubstrate comprises bonding the polymeric cathode layer to the polymericelectrolyte layer.
 48. The method of claim 47, wherein forming ahydrogen diffusion barrier on a second surface of the polymer substratecomprises dipping the storage tank into a solution of a monomericprecursor to the polymeric cathode layer and polymerizing the monomericprecursor to form the polymeric cathode layer.
 49. The method of claim42, wherein forming a hydrogen diffusion barrier on a second surface ofthe polymer substrate comprises spraying a solution of a monomericprecursor to the polymeric anode layer and polymerizing the monomericprecursor to form the polymeric anode layer.
 50. The method of claim 42,wherein forming a hydrogen diffusion barrier on a second surface of thepolymer substrate comprises spraying a solution of a monomeric precursorto the polymeric electrolyte layer and polymerizing the monomericprecursor to form the polymeric electrolyte layer.
 51. The method ofclaim 42, wherein forming a hydrogen diffusion barrier on a secondsurface of the polymer substrate comprises spraying a solution of amonomeric precursor to the polymeric cathode layer and polymerizing themonomeric precursor to form the polymeric cathode layer.
 52. The methodof claim 42, further comprising operably coupling a voltage source tothe polymeric anode layer and the polymeric cathode layer.
 53. A methodof preventing diffusion of hydrogen under pressure from within acontainment vessel, comprising: preventing substantial diffusion ofhydrogen through a substantially hydrogen-impermeable and substantiallyproton-permeable material having a first side exposed to the hydrogenunder pressure and a second side; catalytically dissociating anyhydrogen passing through the substantially hydrogen-permeable materialfrom the first side to the second side thereof into protons andelectrons; transporting the electrons from the second side to the firstside through an electrical circuit comprising an applied voltage;transporting the protons from the second side to the first side throughthe substantially hydrogen-impermeable and substantiallyproton-permeable material; and catalytically reassociating the protonsand electrons into hydrogen.